1. Field
The present invention relates to a high saturation optical output power.
2. Description of the Related Art
To cope with a drastic increase in communication needs in recent years, a so-called wavelength division multiplexing (WDM) system is progressively introduced which enables a large capacity transmission with one optical fiber by multiplexing a plurality of signal lights with different wavelengths. In this WDM system, the optical power of an optical signal attenuates due to a loss in each of optical components used for multiplexing and demultiplexing optical signals with different wavelengths, and therefore use of an optical amplifier is necessary to compensate the attenuation.
A semiconductor optical amplifier (SOA) is expected as an optical amplifier for compensating the loss of the WDM system because it is small in size and can be designed to obtain a gain in a wide wavelength range.
Since an optical fiber used in a general communication system has no polarization mode for an optical signal, the SOA needs to have an inter-polarization gain difference suppressed to a low value. The SOA also needs to be used in a linear region with the saturation optical output power being sufficiently increased because it causes, when used in a saturation region, a transmission penalty due to deterioration in waveform and crosstalk between wavelength channels caused by the pattern effect.
As an SOA realizing such a low polarization dependent gain and high saturation optical output power at the same time, an SOA using a GaInAs bulk structure with an added tensile strain as an active layer is disclosed in Japanese Patent Application Laid-open No. 2001-53392. Here, using a later structure of a GaInAs bulk active layer having a film thickness of 50 nm sandwiched at its both sides between light confinement layers each having a film thickness of 100 nm, and introducing a strain of −0.25% (in other words, a tensile strain of 0.25%) into the active layer when the active layer width is 1.4 μm, whereby a semiconductor optical amplifier is realized which has a fiber to fiber gain of 19 dB, a polarization dependent gain of 0.2 dB or less, and a fiber coupled saturation optical output power of +17 dB to a signal light having a wavelength of 1550 nm with an element of 1200 μm with an injection current of 500 mA.
In the above-described SOA in Japanese Patent Application Laid-open No. 2001-53392, the active layer is thinned to intend an increase in the saturation optical output power and to aim an increase in the mode cross-section area. However, when the active layer is thinned, the flatness of the active layer cross section is increased to cause a large difference in light confinement factor between polarizations, and therefore a tensile strain is added to the active layer for the purpose of canceling the difference so as to suppress the polarization dependent gain to low.
In this SOA, however, the introduction of the tensile strain into the active layer can suppress the polarization dependent gain to low but brings about a problem of the gain peak wavelength reducing in wavelength. Further, in this case, it is necessary to decrease the film thickness of the active layer in order to achieve the high saturation optical output power, thereby causing a reduction in wavelength of the gain peak wavelength to appear more prominently due to the band filling effect caused by an increase in carrier density.
FIG. 1 shows the injection current dependence of the gain peak wavelength in the SOA using the conventional GaInAs bulk structure for the active layer.
As shown in the diagram, an increase in the injection current into the active layer causes a reduction in the gain peak wavelength due to the band filling effect, the gain peak wavelength shifting to the short wavelength side up to 1455 nm at a current of 500 mA.
FIG. 2 shows the wavelength dependence of the fiber to fiber gain in the SOA using the conventional GaInAs bulk structure for the active layer.
Since C-band (a wavelength of 1530 nm to 1560 nm) and L-band (a wavelength of 1570 nm to 1610 nm) that are wavelength bands used in a standard WDM system are located on a long wavelength side of the gain peak wavelength, the gain substantially lowers on the long wavelength side. In particular, the gain is 28 dB at a wavelength of 1460 nm, whereas the gain is 16.5 dB at a wavelength of 1560 nm and the gain is 8.5 dB at a wavelength of 1610 nm.
Thus, a problem is to satisfy both polarization independence and high saturation optical output power of the SOA as well as to obtain a high gain in C-band and L-band used in an optical fiber communication system, especially in L-band.